CD8+ T lymphocytes are essential players in mounting protective cellular immune responses against pathogens and malignantly transformed cells. Depending on the nature of antigen challenge, CD8+ T cells are equipped with certain levels of plasticity to maximize reduction of infected or tumor cells. After pathogen clearance resulting from acute infections, memory CD8+ T cells persist for long term and provide enhanced protection against the same or related pathogens, and these features constitute the basis for prophylactic vaccines. On the other hand, antigen persistence, as a result of chronic infection and cancers, causes CD8+ T cell exhaustion or dysfunction; nonetheless, the exhausted CD8+ T (Tex) cells can be partly reinvigorated, as seen in recent success of checkpoint blockade in cancer immunotherapy. Understanding molecular circuits that underlie T cell memory and exhaustion is critical for fully harnessing the potentials of cytotoxic CD8+ T cells for improving viral or cancer immunity. In this competitive renewal application, our goal is to uncover the uncharted links of Tcf1 and its HDAC activity with chromatin accessibility and chromatin looping-based enhancer-promoter interactions in memory and exhausted CD8+ T cells. The specific aims are as follows: Aim 1. To investigate how Tcf1 preprograms enhanced recall responses by memory CD8+ T cells. Memory CD8+ T cells exhibit enhanced recall response by more robust proliferation and more rapid activation of cytolytic activities than nave T cells. A major knowledge gap remains regarding the molecular basis and molecular regulators that underlie the heightened immune response. We hypothesize that Tcf1 preprograms central memory CD8+ T cells for their enhanced responsiveness to recall stimulation. We will use unbiased systems biology approaches including DNase-seq and HiC to define how Tcf1 and its HDAC activity control chromatin accessibility and looping. Combined with comprehensive functional studies, we will perform in-depth dissection of the unique molecular wiring that dictates enhanced recall responses by Tcm cells. Aim 2. To determine the capacity of Tcf1 to enhance functional restoration of exhausted CD8+ T cells. Recent advances revealed that Tex cells elicited by chronic infection contain a CXCR5+Tim3? subset that has stem cell-like self-renewing capacity and expresses an elevated level of Tcf1. We hypothesize that Tcf1- dependent regulatory circuits can be utilized to durably enhance Tex functional restoration. We will test various Tcf1 forms and cofactor combinations in chronic viral infection and tumor models, and determine their impact on chromatin accessibility/looping and its link to favorable functional output by Tex cells. This proposal will mechanistically elucidate how Tcf1 orchestrates 3D-genome to program memory CD8+ and exhausted CD8+ T cells for enhanced functional output, and provide timely, much needed insights into devising more effective vaccines and therapeutics for infectious diseases and cancers.